(261d) Characterization of Single Atom Catalysts Using X-Ray Absorption Spectroscopy: Strengths and Pitfalls

In a single atom catalyst (SAC), single atom alloy (SAA) catalyst, or single-site catalyst, it is imperative to conclusively demonstrate that the catalyst does indeed contain the desired species before any claims can be made regarding any unique catalytic performance that is related to the presence of the single atom/site. The most common methods that are utilized for such characterization are aberration corrected scanning transmission electron microscopy (AC-STEM), X-ray absorption spectroscopy (XAS), and FTIR of adsorbed CO. There have also been reports of using CO oxidation kinetics to quantify the fraction of single atoms present in an Ir/MgAl₂O₄ catalyst. The widespread use of XAS for characterizing the diverse array of single atom catalysts, e.g. heterogeneous-, homogeneous- and electro-catalysts, is centered on: (i) its elemental specificity, (ii) that it can be performed in-situ/operando, and (iii) that it can readily be used at the low concentrations of the given element many single atom catalysts contain. Moreover, XAS is the only technique that has the ability to provide atomic-level structural (from modeling of the extended X-ray absorption fine structure, EXAFS) and electronic (from X-ray absorption near edge structure, XANES) information, and how these evolve under reaction conditions. This presentation will discuss the strengths and potential pitfalls of using XAS to confirm the single-atom nature of the active site, using examples both from our own work, and from the literature, to illustrate key points.